Normal lung function requires the presence of adequate amounts of functional alveolar surfactant. When the lung is in a steady-state the rates of release and removal of surfactant must be equal. Several studies suggest that one pathway for the clearance may be recycling of surfactant from the alveoli back into the type II cell. The overall goal of this project is to study some of the factors involved in recycling. Recent studies in our laboratory suggest that alveolar surfactant obtained by lung lavage can be divided by differential centrifugation into subfractions which appear to be in a metabolic sequence. The heavier, more dense material appears to be a precursor to the lighter, less dense material. The heavier ("new") material contains some tubular myelin and vesicles and is surface active. The lighter, less sedimentable ("old') material contains vesicular structures and is relatively inactive. A model consistent with these findings is that tubular myelin, formed from newly released lamellar body material, generates a surface film which is eventually reduced to vesicular structures small enough to be endocytosed back into type II cells. This endocytosed material is recylced. I am proposing that the lighter, less dense material is the subfraction of surfactant which is preferentially recycled back intotype II cells. I am also proposing that surfactant apoproteins serve as ligands which can mediate the endocytosis. I propose to test this hypothesis with three specific aims. 1) To determine if an old subfraction of surfactant is taken up into the lung faster than newly released material. These experiments will be done by measuring the rates of uptake of radioactively labeled new and old surfactant given to intact animals and isolated type II cells. 2) To determine what proteins are present in the subfractions of new and old surfactant. Electrophoresis and antibodies will be used to identify the proteins of the subfractions. 3) To determine whether the proteins of the old or new material relate to the uptake of phospholipids. Proteins of the subfractions will be labeled with radioactive amino acids and the rates of uptake of the proteins will be compared.